AQA GCSE PHYSICS · PAPER 1 · FOUNDATION & HIGHER
Nuclear equations
Atomic structure · Lesson 5 of 10
Warm-up
Answer each one, then check.
1. What is the mass number?
Protons plus neutrons
2. What is an alpha particle?
Two protons and two neutrons
3. What is a beta particle?
A fast electron from the nucleus
4. What is the atomic number?
Number of protons
5. Does gamma change the nucleus's mass or charge?
No
Learning Objectives
1. Use the symbols for common nuclei and particles.
2. Balance mass numbers and atomic numbers in alpha and beta decay equations.
3. Describe the effect of alpha, beta and gamma emission on the nucleus.
Nuclear Equations
Alpha: ⁴₂He. Beta: ⁰₋₁e. In a balanced nuclear equation the mass numbers and the atomic numbers are the same on both sides.
Alpha decay: mass number falls by 4, atomic number falls by 2. Beta decay: mass number unchanged, atomic number rises by 1. Gamma: no change to mass or charge.
Alpha and Beta Decay Equations
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Add up the top numbers and the bottom numbers on each side. |
Balanced nuclear equations for the alpha decay of uranium-238 and the beta decay of carbon-14.
Effects on the Nucleus
How the numbers change.
|
Emission |
Mass number |
Atomic number |
|---|---|---|
|
Alpha (α) |
Decreases by 4 |
Decreases by 2 |
|
Beta (β) |
Stays the same |
Increases by 1 |
|
Gamma (γ) |
Stays the same |
Stays the same |
Alpha Decay
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Radium-226 (²²⁶₈₈Ra) decays by alpha emission. Complete: ²²⁶₈₈Ra → ^A_ZRn + ⁴₂He |
1. Mass numbers
226 = A + 4, so A = 222
2. Atomic numbers
88 = Z + 2, so Z = 86
Answer: ²²⁶₈₈Ra → ²²²₈₆Rn + ⁴₂He
Beta Decay
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Carbon-14 decays by beta emission. Complete: ¹⁴₆C → ¹⁴_ZN + ⁰₋₁e |
1. Mass numbers
14 = 14 + 0, so balanced
2. Atomic numbers
6 = Z + (−1), so Z = 7
Answer: ¹⁴₆C → ¹⁴₇N + ⁰₋₁e
Why It Works
Understand the changes.
▸ Alpha. Two protons and two neutrons leave the nucleus.
▸ Beta. A neutron turns into a proton and a fast electron is emitted.
▸ Gamma. The nucleus loses energy only.
▸ Checking. Add up the top numbers, then the bottom numbers, on each side.
Key Terms
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Nuclear equation An equation showing what happens to a nucleus in radioactive decay. |
Balanced The same mass number total and atomic number total on both sides. |
|
Daughter nucleus The nucleus formed after decay. |
Alpha decay Emission of a helium nucleus. |
|
Beta decay A neutron changes to a proton and an electron is emitted. |
Gamma emission Release of energy from the nucleus as electromagnetic radiation. |
Your Task: Balance the Equation
12 minutes
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Complete: (a) ²³⁸₉₂U → ^A_ZTh + ⁴₂He (b) ⁹⁰₃₈Sr → ⁹⁰_ZY + ⁰₋₁e 1. Balance the top numbers. 2. Balance the bottom numbers. |
A good answer shows: (a) A = 234, Z = 90. (b) Z = 39.
Can I...?
☐ Write the symbol for alpha.
☐ Write the symbol for beta.
☐ Balance mass numbers.
☐ Balance atomic numbers.
☐ Complete an alpha decay equation.
☐ Complete a beta decay equation.
☐ State the effect of gamma emission.
☐ Explain beta decay.
Summary
✓ Alpha decay: mass −4, atomic −2.
✓ Beta decay: mass unchanged, atomic +1.
✓ Gamma: no change.
✓ Balance both rows of numbers.
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EXAM FOCUS Complete ²²⁶₈₈Ra → ^A_ZRn + ⁴₂He. (2 marks) Balance the top and bottom numbers. |